Branchial gas transfer, acid-base balance, and hemodynamics were critically evaluated and compared in Ringer-perfused and blood-perfused heads of rainbow trout. Blood perfusion stimulated O2 uptake and CO2 excretion across the gills to values more representative of intact fish. The stimulatory effect of blood on gas transfer was due to increased O2 carrying capacity (O2 uptake) and the presence of erythrocytic carbonic anhydrase (CO2 excretion). Adding carbonic anhydrase to Ringer enhanced CO2 excretion in a manner similar to blood. During Ringer perfusion, arteriovenous pH gradients were abnormal (arterial pH less than venous pH). Perfusion with blood or addition of carbonic anhydrase to Ringer reversed the pH gradients to typical in vivo levels. Branchial vascular resistance to flow was abnormally high in both Ringer- and blood-perfused preparations, primarily as a result of low dorsal aortic pressure. Input pressure increased during blood perfusion and was similar to ventral aortic pressure in vivo. Perfusion with Ringer may have caused irreversible deterioration of gill function as indicated by decreased arterial Po2 and O2 extraction effectiveness after a rapid switch from Ringer to blood perfusion. The results are discussed with reference to the suitability of perfused trout head preparations for studying gill gas transfer, acid-base balance, and hemodynamics. Comparisons are made between the perfused head preparation and intact fish as well as with other types of perfused gill preparations.